The demand on today's networks is increasing in what has been dubbed as the information age. The need for networks with higher capacities at lower costs are being fueled by the growth of the Internet and the World Wide Web as well as the number of business and residential customers utilizing high speed networks for day to day functions.
Control signals are used in such networks for a variety of functions. As an example, in circuit switched or based networks, signaling channels are used as a type of control signal for network control and configuration, network management, routing protocols and network monitoring. In packet switched or based networks, packet headers or labels are used as type of control signal to direct or route packets to their destination. In order to keep the costs associated with processing control signals in high-speed networks low (whether they are circuit-switched or packet-switched), one of the goals is to be able extract only the control signal at intermediate nodes without having to also extract the entire high-speed data stream.
A solution to the demand for higher speed networks is an optical network. Optical networks utilize optical fibers which offer higher bandwidths and are less susceptible to electromagnetic interference and other undesirable effects. Control signals in optical networks may be transmitted “out-of-band” or “in-band”. In “out-of-band” methods, control signals are placed outside of the channel's optical filter bandwidth. For example, in out-of-band signaling such control signals may be placed on a separate wavelength than that of the data channel. An example of this is the optical supervisory channel (OSC) commonly used in dense wavelength division multiplex (DWDM) systems.
In “in-band” communications, the control signal is maintained within the optical filter bandwidth maintained for transporting a particular channel through a network. The most common approach to an “in-band” control signal is to time division multiplex (TDM) control information along with the data. Other “in-band” techniques include subcarrier multiplexing (SCM), amplitude modulated tones, and differential phase-shift keying (DPSK) envelopes. These techniques have significant drawbacks, particularly when scaling to high data rates. As data rates increase, the reading and writing hardware for TDM and SCM schemes become complex. In addition, to be able to rewrite control information as the data passes through network elements generally requires the use of a laser source to perform the erasure and rewriting process for the majority of the developed “in-band” control schemes. In addition, the TDM and DPSK methods typically require substantially tight synchronization between the data and control signal at high data rates.
For instance, with regard to high frequency SCM, in order to implement this technique, high frequency electronics are required since the label spectrum (i.e., the optical bandwidth the label occupies) must be positioned above the highest frequency component in the data spectrum. Also, a power penalty is introduced due to a SCM modulation index. There are also issues with control signal erasure and rewrite. For instance, for control signal erasure, high frequency SCM requires the use of an optical filter or a non-transparent regenerator/wavelength converter. Control signal rewrite requires the use of a dedicated laser followed by remodulation. Such techniques are also difficult to integrate with high payload rates. Additionally, high frequency SCM resembles an “out-of-band” technique when, in fact, it presents dispersion issues of an ultra-wideband signal.
As another example of an “in-band” communication with drawbacks, high bit rate header methods present their own problems. In a high bit rate header method, high speed processing is required since the control signal is operating at a rate comparable to the data channel rate. There may also be challenges with regard to reading, erasing and rewriting of control signals. For example, reading control signals may require high speed demultiplexing or optical correlation. Erasing control signals may require the use of high speed optical gating and nonlinear methods for optical limiting. Control signal rewriting may require a laser and high speed modulation. The timing may also be critical in control signal rewriting.
The use of DPSK as a control signal requires high levels of coherency in the optical signal in order to have proper constructive and/or destructive interference at the receiver's DPSK demodulator. As such, the DPSK method is not transparent to signal format. The DPSK method may also have issues regarding control signal reading, erasing and rewriting. For example, reading control signals may require phase-sensitive detection. Erasing control signals may require the use of a non-transparent regenerator/wavelength converter. Control signal rewriting may require a dedicated laser followed by remodulation.